List View: Allow horizontal scroll in panels in the post editor

[cawa-93]

What?

Fix #49610

[github-actions]

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[andrewserong]

Thanks for fixing up this issue @cawa-93. I've taken this for a spin and it appears to be testing nicely:

✅ With a heavily nested list view, once the list view items go past the right edge of the container, a horizontal scrollbar is introduced
✅ The document outline view is unaffected in its empty headings state
✅ The document outline view is unaffected in a post with lots of long headings at various levels

Scrollbar introduced	When scrolled right, block settings are now clickable

There are likely further follow-ups to be made to improve how deeply nested lists behave (e.g. the adjustments to spacing over in #49600), but for now, I think this is a good step to ensure the block settings buttons are accessible.

LGTM 👍